There are two common methods of controlling the temperature of air discharged from a temperature controlled heating, ventilation and air-conditioning (HVAC) unit used in vehicles such as passenger cars and trucks and commercial trucks and vehicles. In the first method, commonly referred to as air mix systems, engine coolant is continuously circulated through a heater core connected to the vehicles engine. The air temperature discharged from the HVAC unit is controlled by directing a portion of the total system airflow coming from the air-conditioning evaporator core through the heater core. The volume of air passing through the heater core is mixed with the volume of air bypassing the heater core to obtain the desired air temperature. The volume of air flowing through the heater core and thus the discharge air temperature is controlled by adjusting the position of an air mix door that regulates the volume of air flowing through the heater core.
In the full cold position, the air mix door directs all of the airflow to bypass the heater core, thus producing the coldest discharge air temperatures. In the full hot position, the air mix door directs all of the airflow to pass through the heater core, thus producing the hottest discharge air temperature. In intermediate positions, varying volumes of airflow are directed through the heater core to achieve the desired discharge air temperature.
In the second type of HVAC unit, commonly referred to as a fluid controlled system, the entire volume of air flowing through the air-conditioning evaporator is passed through the heater core. The temperature of the air discharged from the HVAC unit is controlled by regulating the volume of engine coolant passing through the heater core, thus regulating the temperature of the heater core. The volume of engine coolant supplied to the heater core is controlled by a temperature regulated fluid control valve.
In the full cold position, the fluid control valve prevents any engine coolant from flowing through the heater core, producing the coldest temperature, while in the full hot position, the fluid control valve allows a continuous volume of engine coolant to flow through the heater core, producing the hottest temperature. To obtain intermediate discharge air temperatures the fluid control valve is opened or closed for differing periods of time to regulate the volume of engine coolant passing through the heater core, thereby regulating the temperature of the heater core and discharge air.
Both air mix and fluid controlled HVAC units have attendant advantages and disadvantages. Properly designed air mix HVAC units can produce changes in the system discharge air temperature faster than fluid controlled HVAC units of similar size. Air mix HVAC units also produce greater volumes of air because all the air volume does not pass through the flow restrictive heater core. However, properly designed air mix HVAC units are more difficult to design and build and create dimensional restraints on the size and placement of system components. Fluid controlled HVAC units produce changes in discharge air temperature slower than air mix HVAC units but are simpler to design and allow a greater range of component placement while still producing an effective HVAC system.
The fluid control valves used in current fluid controlled HVAC units are complex, expensive to produce, and prone to maintenance problems. These problems have been most prevalent in larger HVAC units that operate under greater system pressures and higher coolant volumes, for example, HVAC units on commercial trucks, buses, etc.
Thus, there exists a need for a fluid control valve that reduces or eliminates the problems of prior HVAC fluid control values. Specifically, such a fluid control valve should reduce the complexity, maintenance, and possibly cost of the control valves.